1. Field of the Invention
This invention relates to radiant energy, and more particularly to radiant energy responsive electric signalling whether visible or invisible energy is involved.
2. Description of the Prior Art
A published letter entitled "Enhancement of Transverse Thermoelectric Voltages in Thin Metallic Films" by von Gutfeld and Caswell in Applied Physics Letters, Vol. 25, No. 12, Dec. 15, 1974 p. 691-693 shows a scanning electron micrograph shown here as FIG. 1 which shows the result of evaporation of Mo onto a substrate supported at a 70.degree. angle with respect to the horizontal. The columnar needles form a structure which is clearly visible with columnar needles extending on the order of 1000A in depth and on the order of 300A on a side, separated by approximately the same amount. The columnar needles point in the direction of the incident vapor beam. The columnar structure appears to extend down to the substrate.
In a published letter entitled "Temperature Dependence of Transverse Planar Voltages in Laser-irradiated Pt and Pd Films" by von Gutfeld and Tynan, Applied Physics Letters, Vol. 26, No. 12, June 15, 1976 pp. 680-682, a hypothesis is described as to how slant-angle, vapor-deposited thin films of Mo exposed to laser light produce anisotropic transverse planar voltages. In FIG. 2A hereof taken from that letter it is pointed out that the structure of a slant-angle film can be modeled as a large number of very thin columns of metal parallel with one another. The material is assumed to vary in composition for columns and spaces in between based upon the concept that perhaps more O.sub.2 atoms would be trapped near the base of the region between columns than in the columns themselves. A schematic of a model incorporating a composition and temperature periodicity in the plane of vapor deposition in Shown in FIGS. 2A and 2B respectively. The arrows in FIG. 2A indicate illumination by laser light.
A periodic composition and temperature variation in the transverse direction is shown. The regions A represent the slanted film columns. Base areas B are the spaces between columns. For this structure, the resulting thermopower contributions will also be periodic if, first, there exists an increase in defects such as trapped O.sub.2 in region B compared to region A due to a higher ratio of residual O.sub.2 atoms compared to metal atoms during deposition, and/or, second, there is an increase in size effects for the thinner regions B compared to A.
For the periodic temperature fluctuation shown in FIG. 2A (asymmetric about the midpoint of A) each element AB is equivalent to a microthermocouple and the total transverse voltage will be EQU V = nL(S.sub.A -S.sub.B) (T.sub.1 -T.sub.0), (1)
where n is the number of columns/length, L is the transverse length illuminated by the laser, and S.sub.A and S.sub.B are the thermopowers of the regions A and B, respectively. Periodic transverse temperature fluctuations have been inferred using an experimental electrostatic analog method. The value T.sub.1 -T.sub.0 is measured at the base of A, from the left to the right in FIG. 2A. T.sub.1 is larger than T.sub.0 as shown since the point is closer to the end of the needle exposed to radiation R. T.sub.1 -T.sub.0 is found to be about 5% of the temperature difference between the top and base of the column for columns inclined 60.degree. from the normal. For zero inclination, T.sub.1 -T.sub.0 and the resulting V are both zero. Equation (1) predicts values of V consistent with those observed at room temperature and near the phonon drag peaks for S.sub.A -S.sub.B (.DELTA.S) on the order of 1 .mu.V/.degree.K for Pt. At room temperature, this difference determined only from size effect measurements on Pt gives rise to a .DELTA.S.about.0.7 .mu.V/.degree.K for Pt films. The experimentally determined room-temperature change in the bulk thermopower of vanadium due to O.sub.2 is .apprxeq. 0.3 .mu.V/(.degree.K at. %0.sub.2). A similar order-of-magnitude change can be expected in Pt in region B. A quantitative estimate for .DELTA.S at low temperatures is difficult due to a lack of data for very thin films. Generally, defect scattering will decrease the usual phonon drag thermopower. This decrease should occur especially in region B.
The result is that the columns appear to be a series of microscopic thermocouples connected in a very long series array, thereby producing a substantial voltage.
U.S. Pat. No. 3,851,174 of Tynan and von Gutfeld, assigned to International Business Machines Corporation, filed May 4, 1973 and entitled "Light Detector for Nanosecond-DC Pulse Width Range" describes a dielectric substrate coated with a thin film deposit of a high melting point metal such as Mo or W exhibiting an anisotropy. A pair of electrical contacts is electrically connected at two points on the thin film deposit. A laser pulse is applied to the surface of the thin film deposit, resulting in a voltage pulse across the contacts, proportional in magnitude to the incident pulse.
In a published letter entitled "Phonon-induced planar voltages in thin metallic films," by von Gutfeld, Tynan and Budd, Applied Physics Letters, Vol. 2, January 15, 1976, pp. 78-80, it is shown that any energy producing phonons (i.e. quantized elastic waves or lattice vibrations) such as light, heat or any other thermal source will produce a transverse voltage in slant-angle deposited thin films. Previously, semiconductor films produced at slant angles and optically irradiated had been reported to be photoelectric in nature. The subject letter clarified the fact that what is involved in metallic films is a thermal rather than optical effect.
U.S. Pat. No. 3,697,709 of Dreyfus, assigned to International Business Machines Corporation, describes a beam addressable storage and display tube in which each localized storage element has associated therewith an erase element which is energized by a beam of input energy. The output energy of the erase element is coupled into the associated storage element to affect its state. Thus, this reference shows the use of two adjacent layers of material, one of which is struck by an input beam (such as a light beam) and produces energy which is generally heat or light. This heat energy or light energy directly couples into the associated adjacent storage element to change the state of the storage element. However, this reference does not teach or suggest associated elements of the same material, and especially those exhibiting a transverse thermoelectric effect.
Heretofore, it has been attempted to provide enhanced output by increasing the thickness of slant-angle films. However, it has been found that increasing thickness leads to closure of the spaces between the columns of metal, tending to eliminate the thermoelectric effect altogether. Once the film is about 1500A thick, further thickness causes no change in the voltage up to about 5000A while thicknesses greater than 5000A lead to the above-mentioned decrease in voltage due to closure of spaces.